Proper maintenance of process equipment used for manufacturing, shipment, storage, and various handling procedures is important for the equipment to operate reliably, safely, and economically. Often the integrity of process equipment is monitored to detect any corrosion or other degradation. The need to monitor process equipment integrity becomes more acute when the equipment is subjected to a highly corrosive environment or when the process equipment contains especially hazardous materials. Such an environment may result from aggressive processes such as chlorine, oxidizer, acid, and/or base manufacturing, or other such processes that are known to pose challenges to operators to avoid destroying process equipment and/or loss of containment. Quick and accurate assessments of corrosion of a material in a highly corrosive environment is thus useful in maintaining reliable and safe operation of the equipment and in some cases is required by law.
Monitoring techniques have been used to determine an average corrosion rate for a material over an extended period of time. Some monitoring techniques, such as those using ultrasonic technology, perform a determination of corrosion from outside the process equipment, e.g., thickness measurements are used to infer metal loss, and thus the amount of degradation or corrosion. Determining an average corrosion rate over an extended period of time does not provide real-time information regarding the degradation of a material, however. Rather it is the integration of a number of corrosion events over time. Furthermore, determining the corrosion from outside the process equipment may compromise accuracy of the estimated corrosion condition inside the equipment. These ex-situ methods often also involve placing personnel near the measurement point, sometimes at high elevations within the process equipment, which can place the personnel in a dangerous situation.
Electrical resistance (ER) probes have been used to measure a corrosion rate inside the process equipment. ER probes use a highly deformed piece of metal of a composition similar to the material of interest, and determine a corrosion rate of the target material based on proportionality of the change in resistance of the deformed metal to the corrosion of the target material. ER probes are highly sensitive to temperature change, resulting in poor accuracy. Further, ER probes are generally very thin, which both limits the dynamic range of measurement and results in a short sensor life. Highly deformed metals also corrode differently than metals of the same composition, but with less mechanical deformation. Additionally, ER probes do not address non-conductive materials.
Coupons have also been used for corrosion monitoring inside process equipment. A coupon is a small piece of metal, which is attached to the inside of the process equipment. The mass of a coupon is determined prior to and after exposure to a corrosive environment over a long period of time (nominally years). The estimated corrosion rate is thus the integration of corrosion events over a period of time; singular corrosion events are not identified. One difficulty in using coupons is the need to remove adherent corrosion products from the exposed coupon, prior to final weighing. One needs to be careful that the removal process does not bias the end result. Additionally, a coupon may be consumed without any indication prior to removal and inspection. Without real-time corrosion measurement capability, it is difficult to make improvements or to identify process variables related to the corrosion event.
Electrochemical devices have also been used for corrosion monitoring inside process equipment. Typically, corrosion of metals is electrochemical in nature, so the corrosion rate may be measured by use of electrochemistry, e.g., the transfer of electrons from a cathode to an anode. This process assumes the corrosion mechanism and a voltage/current, V/I, are both known, and these assumptions are easily incorrect because of complexity in electrochemical calculations.